This year, working in close collaboration with collaborators, we had three major accomplishments in the mismatch repair field. (1) We completed experiments indicating that exonuclease 1, which participates in the excision step of MMR, is also important for cellular responses to DNA damage induced by environmental agents. (2) We finished experiments demonstrating that Saccharomyces cereivisiae MutL alpha is a mismatch repair endonuclease. (3) We directly visualized asymmetric adenine nucleotide induced conformational changes in MutL alpha, leading to a better understanding of the role of this heterodimer in mismatch repair.